Integrated photodetector waveguide structure with alignment tolerance

ABSTRACT

An encapsulated integrated photodetector waveguide structures with alignment tolerance and methods of manufacture are disclosed. The method includes forming a waveguide structure bounded by one or more shallow trench isolation (STI) structure(s). The method further includes forming a photodetector fully landed on the waveguide structure.

FIELD OF THE INVENTION

The invention relates to semiconductor structures and, moreparticularly, to encapsulated integrated photodetector waveguidestructures with alignment tolerance and methods of manufacture.

BACKGROUND

Photosensors or photodetectors are sensors that detect light or otherelectromagnetic energy. There are several varieties of photosensors orphotodetectors, many of which are manufactured using conventional CMOStechnologies. For example, photosensors or photodetectors can be activereceivers commonly used in photonic integrated circuit (PIC)transceivers. These types of transceivers have emerged as an alternativeto transceivers that use discrete opto-electronic components.

Many types of photosensors or photodetectors implement CMOS integratednanophotonics circuits. These nanophotonics circuits include crystallinematerials like germanium or III-V compounds, which are desirable for useas the active element in photodetector components. This is due to theirhigh quantum efficiency. In the manufacturing process, the crystallinematerials are encapsulated in order to protect the crystalline structurefrom other manufacturing processes.

Using rapid melt growth, amorphous or polycrystalline films (e.g.,germanium or III-V compounds) can be deposited at low temperatures in anamorphous or polycrystalline state, and then crystallized thermally.This technique provides for a high degree of integration flexibility.During the crystallization anneal, though, the amorphous orpolycrystalline material (e.g., Ge) expands and contracts, creatingstress on the encapsulation films. These stresses can be exacerbated dueto the encapsulation films being formed on non-planar surfaces such asdivots formed by processing of shallow trench isolation (STI)structures.

These stresses can create a breach in the encapsulation, resulting indefects that can subsequently degrade the operation of thephotodetector. For example, light coupled to extrusions can result inslow diffusion of carriers to contacts, thereby limiting a 3 dbbandwidth of the detector.

SUMMARY

In an aspect of the invention, a method comprises forming a waveguidestructure bounded by one or more shallow trench isolation (STI)structure(s). The method further comprises forming a photodetector fullylanded on the waveguide structure.

In an aspect of the invention, a method comprises forming an opticalwaveguide having a first lateral boundary and a second lateral boundary,bounded by pull down structures. The method further comprises forming afirst encapsulating layer fully landed on the optical waveguide, with awindow exposing a surface of the optical waveguide. The method furthercomprises forming photodetector material within the first lateralboundary and second lateral boundary of the optical waveguide, on thefirst encapsulating layer and in contact with the surface of the opticalwaveguide through the window. The method further comprises forming asecond encapsulating layer over the photodetector material and withinthe first lateral boundary and the second lateral boundary of theoptical waveguide. The method further comprises crystallizing thephotodetector material to form a photodetector fully landed on theoptical waveguide.

In an aspect of the invention, a sensor structure comprises: a waveguidestructure bounded by one or more shallow trench isolation (STI)structures; and a photodetector fully landed on the waveguide structure,adjacent to the one or more STI structures.

In another aspect of the invention, a design structure tangibly embodiedin a machine readable storage medium for designing, manufacturing, ortesting an integrated circuit is provided. The design structurecomprises the structures of the present invention. In furtherembodiments, a hardware description language (HDL) design structureencoded on a machine-readable data storage medium comprises elementsthat when processed in a computer-aided design system generates amachine-executable representation of the encapsulated integratedphotodetector waveguide structures with alignment tolerance, whichcomprises the structures of the present invention. In still furtherembodiments, a method in a computer-aided design system is provided forgenerating a functional design model of the encapsulated integratedphotodetector waveguide structures with alignment tolerance. The methodcomprises generating a functional representation of the structuralelements of the encapsulated integrated photodetector waveguidestructures with alignment tolerance.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIGS. 1-4 b show processing steps and respective structures inaccordance with aspects of the present invention;

FIGS. 5-9 show various structures and related processing steps inaccordance with various aspects of the present invention; and

FIG. 10 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

The invention relates to semiconductor structures and, moreparticularly, to encapsulated integrated photodetector waveguidestructures with alignment tolerance and methods of manufacture. Morespecifically, the present invention is directed to encapsulatedrecrystallized amorphous/polycrystalline/polysilicon material sensorsand methods of manufacture. In embodiments, the sensors can be anyamorphous/polycrystalline/polycrystalline materials such as germanium orIII-V compounds which, upon thermal anneal, will crystallize.

Advantageously, the present invention provides sensors that eliminate orsubstantially reduce defects, which would otherwise be formed duringcrystallization processes. This is accomplished by forming the sensorsfully on a planar surface of the optical waveguide, e.g., underlyingsilicon material. In this way, the sensors will not be located on anydivots, which may be formed at boundaries of STI structures.

By way of background, there is a requirement in silicon photonics tointegrate a photodetector and waveguide together, with the photodetectorlocated above the waveguide. There is also a need to generate taperedphotodetector shapes to improve light collection efficiency. Theseshapes, however, create stress during the germanium crystallization,resulting in material extrusions and, in turn, lower 3 db bandwidth dueto slow diffusion of photo generated carriers to the contacts. Thepresent invention eliminates or substantially reduces defects in thephotodetector by landing the photodetector fully on a planar surface ofthe optical waveguide, e.g., underlying silicon material. Morespecifically, in embodiments, the sensors of the present invention arefully landed on silicon, eliminating any overlay issues and thereforeeliminating additional stresses which may cause problems of extrusion ofgermanium or other material.

In embodiments, the photodetector structure of the present inventioncomprises silicon on insulator (SOI) material forming an opticalwaveguide having a first side and a second side, bounded by shallowtrench isolation (STI) structure(s). A first dielectric layer isdeposited on the optical waveguide to encapsulate a bottom surface of asubsequently deposited photodetector material, e.g., Ge material. Asshould be understood by those of skill in the art, the dielectric layerbetween the optical waveguide and photodetector material can serveseveral purposes: (i) it can confine the crystallization seed to awindow defined by an opening to SOI in this layer, (ii) it forms abottom encapsulation layer of the photodetector, and (iii) it is thelight coupling layer between the optical waveguide and thephotodetector.

The photodetector, which can have a tapered input end or other geometryas described herein, is formed on the first dielectric layer, such thatthe dimensions of the photodetector lie within a lateral boundary of theoptical waveguide. That is, the lateral boundary of the photodetectorwill not extend onto adjacent STI structures or divots that may be inthe STI structures, at a juncture of the STI structures and opticalwaveguide. A second dielectric layer is formed over the photodetector asan encapsulation layer, such that an interface between the photodetectorand the encapsulation layer remains within the lateral boundary of theoptical waveguide. The top dielectric layer, e.g., encapsulation layer,is confined within the boundary of the optical waveguide and will landon the bottom encapsulation layer. This configuration preventsextrusions of the photodetector outside the lateral boundary of thewaveguide due to added stresses.

The sensors of the present invention can be manufactured in a number ofways using a number of different tools. In general, though, themethodologies and tools are used to form small structures withdimensions in the micrometer scale. The methodologies, i.e.,technologies, employed to manufacture the encapsulated sensors of thepresent invention have been adopted from integrated circuit (IC)technology. For example, the structures of the present invention arebuilt on wafers and are realized in films of material patterned byphotolithographic processes on the top of a wafer. In particular, thefabrication of the encapsulated sensors of the present invention usesthree basic building blocks: (i) deposition of thin films of material ona substrate, (ii) applying a patterned mask on top of the films byphotolithographic imaging, and (iii) etching the films selectively tothe mask.

FIGS. 1-4 b show processing steps and respective structures inaccordance with a first aspect of the present invention. Morespecifically, FIG. 1 shows a structure 5 comprising an SOI wafer 10. Inthe SOI wafer implementation, an insulation layer (e.g., BOX) 14 isformed on top of a wafer (bulk substrate) 12, with an activesemiconductor layer 16 (e.g., active silicon) formed on the BOX 14. Inembodiments, the semiconductor layer 16 is a single crystalline activesemiconductor layer.

In embodiments, the constituent materials of the SOI wafer 10 may beselected based on the desired end use application of the semiconductordevice. For example, the BOX 14 may be composed of oxide, such as SiO₂.Moreover, the single crystalline active semiconductor layer 16 can becomprised of various semiconductor materials, such as, for example, Si,SiGe, SiC, SiGeC, etc. The SOI wafer 10 may be fabricated usingtechniques well known to those skilled in the art, e.g., oxygenimplantation (e.g., SIMOX), wafer bonding, etc.

Still referring to FIG. 1, a resist 18 is formed on the activesemiconductor layer 16. The resist 18 is exposed to energy (e.g., light)in order to form a pattern (openings). The active semiconductor layer 16undergoes a patterning process, e.g., etching process, through theopenings to form one or more trenches 20 to the underlying oxide(insulator layer). In embodiments, the trenches 20 can be shallowisolation trenches which define boundaries of the optical waveguide,formed from the active semiconductor layer 16. In embodiments, theresist 18 can be removed by an oxygen ashing process known to those ofskill in the art. One or more pad films can also be provided.

In FIG. 2, the trenches 20 are filled with an insulator material 22. Forexample, in embodiments, the trenches 20 can be filled with an oxidematerial to form one or more STI structures. The oxide material can bedeposited using conventional deposition processes such as chemical vapordeposition (CVD) processes. Any excess oxide material can be removed bya planarization process. Through various processes, though, divots orrecesses (also known as pull downs) 24 form in the STI structure at thejunction between the insulator material 22 and the active semiconductorlayer 16 (optical waveguide). These divots or recesses 24 can causeunwanted stress on the subsequently formed photodetectors.

In FIG. 3, an encapsulating material (e.g., dielectric material) 26 isformed on the active semiconductor layer 16, which has a planar surface,as well as the insulator material 22 and divots or recesses 24. Inembodiments, the encapsulating material 26 can be nitride, oxide orother hardmask materials deposited using conventional CMOS depositionprocesses, e.g., CVD processes. In embodiments, the encapsulatingmaterial 26 can have a thickness of about 500 Å; although otherdimensions are also contemplated by the present invention.

As shown in FIG. 3, in embodiments, the encapsulating material 26 can bepatterned using conventional lithography and etching processes. By wayof non-limiting illustrative example, a resist formed on theencapsulating material 26 can be exposed to energy (light) to form apattern (openings). A reactive ion etching (RIE) is performed on theexposed portions of the encapsulating material 26 to pattern theencapsulating material 26. In embodiments, the patterning results in awindow or opening 26 a which exposes a portion of the underlyingsemiconductor layer 16 (optical waveguide). In embodiments, the window26 a can be fully landed on the active semiconductor layer 16. Theexposed underlying semiconductor layer 16 will act as a seed layer tocrystallize amorphous or polycrystalline material, e.g., Ge or otherIII-V compound material, during subsequent annealing processes. Theresist can then be removed by a conventional stripping process, e.g.,oxygen ashing.

In FIGS. 4a and 4b , amorphous or polycrystalline material 28, e.g., Geor other III-V compound material, is formed on the encapsulatingmaterial 26. More specifically, the amorphous or polycrystallinematerial 28, e.g., Ge or other III-V compound material, is deposited onthe encapsulating material 26 and, if any, exposed underlyingsemiconductor layer 16. As shown in FIGS. 4a and 4b , the amorphous orpolycrystalline material 28, e.g., Ge or other III-V compound material,is patterned to be fully landed on the encapsulating material 26 and,hence, fully landed on the active semiconductor layer 16 (opticalwaveguide), within the boundaries defined by the divots or recesses 24.That is, the material 28 will be adjacent to, but not contacting, thedivots or recesses 24.

As shown in FIG. 4a , the underlying encapsulating material 26 can befurther patterned, e.g., etched, so that it is fully landed on theactive semiconductor layer 16 (optical waveguide), within the lateralboundaries of the optical waveguide as defined by the divots or recesses24. In the embodiment shown in FIG. 4a , the encapsulating material 26will be adjacent to, but not contacting, the divots or recesses 24;however, as shown in FIG. 4b , the underlying encapsulating material 26can be patterned into different configurations such as, for example,extending onto the encapsulating material 26 and insulator material 22(and within the divots or recesses 24) or other dimensions contemplatedby the present invention. In optional embodiments, the encapsulatingmaterial 26 does not need to undergo any additional etching process.

In any of the embodiments, an encapsulating material 26′, e.g., nitride,oxide or other hardmask material, is formed over the amorphous orpolycrystalline material 28. In embodiments, the encapsulating material26′ will land adjacent to or on the encapsulating material 26, to fullyseal the material 28. The encapsulating material 26′ will also landfully on and within the confines of the active semiconductor layer 16(optical waveguide). The encapsulating material 26′ can have a thicknessof about 500 Å; although other dimensions are also contemplated by thepresent invention.

As further shown in FIGS. 4a and 4b , the amorphous or polycrystallinematerial 28 undergoes an annealing process (shown by the arrows) abovethe melting temperature of the material 28, e.g., about 950° C. for Ge.As should be understood by those of skill in the art, the annealingprocess will melt and crystallize the amorphous or polycrystallinematerial 28 thereby forming a photodetector 30, which is now fullylanded on a planar surface of the active semiconductor layer 16. Theanneal process can also be used to anneal source and drain regions,thereby reducing processing steps and manufacturing costs. As thephotodetector 30 is fully landed on a planar surface of the activesemiconductor layer 16, e.g., within the lateral boundaries of theoptical waveguide as defined by the divots or recesses 24, the presentinvention is capable of significantly reducing stress and overlayissues.

FIGS. 5-9 show various structures and related processing steps inaccordance with various aspects of the present invention. In each ofthese structures, the encapsulated material 28, e.g., photodetector 30,is fully landed on the active semiconductor layer 16, e.g., opticalwaveguide structure. That is, the photodetector 30 is within the lateralboundaries of the optical waveguide as defined within the confines ofthe divots or recesses 24. This ensures that the photodetector 30 isfully landed on a planar surface, thereby reducing or eliminating stressthat would otherwise be imposed on the photodetector. As should beunderstood by those of skill in the art, each of the structures formedin FIGS. 5-9 can be fabricated using conventional CMOS processes asdescribed herein.

In FIG. 5, the photodetector 30 includes a tapered input end 30 a. Inembodiments, the tapered input end portion 30 a is a narrowed portionwhich focuses light into the body of the photodetector 30. As in allaspects of the present invention, the photodetector 30 (with the taperedinput end 30 a) is fully landed on the planar surface of the opticalwaveguide 16. In this way, the present invention, e.g., photodetector30, increases operation bandwidth and responsivity.

FIG. 6 shows a multi-mode structure 5′ which allows for displacement ofoptical mode and contacts (not shown) to improve detector responsivity(e.g., less light is scattered or absorbed by contacts prior to creatingelectron-hole pairs). In this implementation, the active semiconductorregion (optical waveguide) 16 includes a narrow portion 16 a, a taperedportion 16 b and a wide portion 16 c. In embodiments, the dimensions ofthe portions 16 a, 16 b and 16 c can vary, depending on the intensityand the wavelength of light. For example, the width of the narrowportion 16 a can be about 0.3 microns and the width of the wide portion16 c can be about 1.0 microns. This implementation also includes thephotodetector 30 with a tapered input end 30 a, fully landed on theplanar surface of the optical waveguide 16. In embodiments, thephotodetector 30 with tapered input end 30 a is shown to be landed onthe tapered portion 16 b and wide portion 16 c of the optical waveguide16; although other configurations are also contemplated by the presentinvention.

In FIG. 7, the multi-mode structure 5″ includes the active semiconductorregion (optical waveguide) 16 with a narrow portion 16 a, a taperedportion 16 b and a wide portion 16 c, as described above; whereas, thephotodetector 30′ is now formed without a tapered input end. In thisembodiment, the photodetector 30′ is fully landed on the planar surfaceof the optical waveguide 16, preferably on the wide portion 16 c;although other configurations are also contemplated by the presentinvention.

In FIG. 8, the multi-mode structure 5′″ comprises an activesemiconductor region (optical waveguide) 16′ and photodetector 30″ bothbeing continuously tapered, e.g., fully tapered. In this implementation,the tapered photodetector 30″ is fully landed on the planar surface ofthe tapered optical waveguide 16′.

In FIG. 9, the multi-mode structure 5″″ comprises a continuously taperedactive semiconductor region (optical waveguide) 16′, and a double(multiple) tapered photodetector 30″. In this implementation, thetapered photodetector 30″ includes a further taper at its input endportion as designated by reference numeral 30 a″. This configurationforms a multiple tapered photodetector. As in previous embodiments, thetapered photodetector 30″ (with tapered input end portion 30 a″) isfully landed on the planar surface of the optical waveguide 16′.

FIG. 10 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test. FIG. 10 shows a block diagram of anexemplary design flow 900 used for example, in semiconductor IC logicdesign, simulation, test, layout, and manufacture. Design flow 900includes processes, machines and/or mechanisms for processing designstructures or devices to generate logically or otherwise functionallyequivalent representations of the design structures and/or devicesdescribed above and shown in FIGS. 1-9. The design structures processedand/or generated by design flow 900 may be encoded on machine-readabletransmission or storage media to include data and/or instructions thatwhen executed or otherwise processed on a data processing systemgenerate a logically, structurally, mechanically, or otherwisefunctionally equivalent representation of hardware components, circuits,devices, or systems. Machines include, but are not limited to, anymachine used in an IC design process, such as designing, manufacturing,or simulating a circuit, component, device, or system. For example,machines may include: lithography machines, machines and/or equipmentfor generating masks (e.g. e-beam writers), computers or equipment forsimulating design structures, any apparatus used in the manufacturing ortest process, or any machines for programming functionally equivalentrepresentations of the design structures into any medium (e.g. a machinefor programming a programmable gate array).

Design flow 900 may vary depending on the type of representation beingdesigned. For example, a design flow 900 for building an applicationspecific IC (ASIC) may differ from a design flow 900 for designing astandard component or from a design flow 900 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc. FIG. 10 illustrates multiple such design structuresincluding an input design structure 920 that is preferably processed bya design process 910. Design structure 920 may be a logical simulationdesign structure generated and processed by design process 910 toproduce a logically equivalent functional representation of a hardwaredevice. Design structure 920 may also or alternatively comprise dataand/or program instructions that when processed by design process 910,generate a functional representation of the physical structure of ahardware device. Whether representing functional and/or structuraldesign features, design structure 920 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 920 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 910 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 1-9. As such,design structure 920 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 910 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1-9 to generate a netlist980 which may contain design structures such as design structure 920.Netlist 980 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 980 may be synthesized using an iterative process inwhich netlist 980 is resynthesized one or more times depending on designspecifications and parameters for the device. As with other designstructure types described herein, netlist 980 may be recorded on amachine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 910 may include hardware and software modules forprocessing a variety of input data structure types including netlist980. Such data structure types may reside, for example, within libraryelements 930 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 940, characterization data 950, verification data 960,design rules 970, and test data files 985 which may include input testpatterns, output test results, and other testing information. Designprocess 910 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 910 withoutdeviating from the scope and spirit of the invention. Design process 910may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 910 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 920 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 990.

Design structure 990 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in an IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 920, design structure 990 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 1-9. In one embodiment, design structure 990may comprise a compiled, executable HDL simulation model thatfunctionally simulates the devices shown in FIGS. 1-9.

Design structure 990 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 990 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIGS. 1-9. Design structure990 may then proceed to a stage 995 where, for example, design structure990: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A sensor structure, comprising: a waveguidestructure; a first encapsulating material deposited directly in contactwith the waveguide structure and including a window exposing a portionof the waveguide structure; a photodetector composed of apolycrystalline material landing on the first encapsulating material anddirectly on the waveguide structure exposed through the window; and asecond encapsulating material encapsulating and surrounding thepolycrystalline material of the photodetector by extending directly onsidewalls of the polycrystalline material and directly in contact withan uppermost surface of the polycrystalline material of thephotodetector.
 2. The sensor structure of claim 1, wherein the waveguidestructure is tapered.
 3. The sensor structure of claim 1, wherein thewaveguide structure is continuously tapered.
 4. The sensor structure ofclaim 3, wherein the photodetector is continuously tapered.
 5. Thesensor structure of claim 4, wherein the continuously taperedphotodetector is fully landed on a planar surface of the continuouslytapered waveguide structure.
 6. The sensor structure of claim 5, whereinthe continuously tapered photodetector is formed as a double taperedphotodetector.
 7. The sensor structure of claim 6, wherein the doubletapered photodetector includes a taper at its input end portion.
 8. Thesensor structure of claim 7, wherein the continuously tapered waveguidestructure is formed from silicon material or silicon on insulatormaterial.
 9. The sensor structure of claim 1, wherein the firstencapsulating material directly contacts a bottom surface of thepolycrystalline material of the photodetector.
 10. The sensor structureof claim 1, wherein the window allows a portion of the polycrystallinematerial to directly contact the waveguide structure.
 11. The sensorstructure of claim 1, further comprising STI structures which bound thewaveguide structure.
 12. The sensor structure of claim 11, wherein thefirst encapsulating material is over the STI structures.
 13. The sensorstructure of claim 12, wherein the polycrystalline material extendsthrough the window to the waveguide structure.
 14. The sensor structureof claim 13, wherein the polycrystalline material directly contacts thewaveguide structure through the window.
 15. The sensor structure ofclaim 14, wherein the first encapsulating material directly contacts abottom surface of the polycrystalline material.
 16. The sensor structureof claim 15, wherein the first encapsulating material extends from thewindow and across a portion of the bottom surface of the polycrystallinematerial.